Signal transmitter for indicating the arrival of mail



FIG. 1 is a perspective plan view showing my new design;

FIG. 2 is a front plan view thereof;

FIG. 3 is a right side plan view thereof;

FIG. 4 is a top plan view thereof; and,

FIG. 5 is a bottom plan view thereof. 

The ornamental design for a signal transmitter for indicating the arrival of mail, as shown and described. 